I can do it - means ‘I have the physical ability, skill, or resources’ that are needed, to do it. Hence ‘it is possible for me to do it’.
I could do it - is as above, but is conditional - which means that ‘doing it’ also depends on the will or choice of the person being called on to do... whatever it is. As in:
‘I could do it...
- if I wanted
- any time after 2.30pm
- if I had time
- if you give me an extra $10
So the ‘could’ is conditional, either on the will or choice of the ‘doer’, or on some other condition (of any kind - weather, time, resources, etc.) being met, in order for the action to be possible.
If you want to ask someone to do something for you, then use ‘could’.
‘Could you park over there, please?’ ‘Yes, sure, no problem’.
If you used can:
‘Can you park over there please?’ ‘Yes I can, but I’m not going to!’
If you use can, you ask about ability.
If you use could, you ask about possibility.